1. Field of Invention
The present invention relates to a power supply device that is suitably used for supplying a predetermined high voltage to a traveling wave tube that is used for amplification and generation of a high frequency signal.
2. Description of the Related Art
A traveling wave tube is an electronic tube for amplification and generation of a high frequency signal by an interaction between an electron beam emitted from an electron gun and a high frequency circuit. As shown in FIG. 1, such the traveling wave tube 1 is a construction that includes: for example, electron gun 10 for emitting electron beam 50; helix 20, which is a high-frequency circuit for causing interaction between electron beam 50 that is emitted from electron gun 10 and a high-frequency signal (microwave); electrode collector 30 for capturing electron beam 50 that is supplied from helix 20; and electrode anode 40 for guiding electron beam 50 that is emitted from electron gun 10 through helix 20.
Electron gun 10 is equipped with: electrode cathode 11 for emitting thermions; heater 12 for supplying thermal energy for causing electrode cathode 11 to emit thermions; and electrode Wehnelt 13 for focusing thermions to form electron beam 50.
A negative high voltage (cathode voltage) is supplied from power supply device 60 to electrode Wehnelt 13 and electrode cathode 11 of electron gun 10, and a predetermined heater voltage Ef is supplied to heater 12 on the basis of the potential of electrode cathode 11. In addition, a positive high voltage (direct-current voltage) is supplied to electrode collector 30 on the basis of the potential of electrode cathode 11. Electrode anode 40 and helix 20 are connected to the case of traveling wave tube 1 that is grounded. Traveling wave tube 1 also includes a configuration in which the connection between electrode anode 40 and helix 20 is cut and different power supply voltages are supplied to electrode anode 40 and helix 20.
In this configuration, electron beam 50, that is emitted from electron gun 10, is accelerated by electrode anode 40 and introduced into helix 20 and then travels inside helix 20 while interacting with the high-frequency signal that is applied as input to helix 20. Output electron beam 50 that is supplied from helix 20 is captured by electrode collector 30. At this time, a high-frequency signal that has been amplified by interaction with electron beam 50 is supplied as output from helix 20.
FIG. 2 is a block diagram showing the configuration of a conventional power supply device. The power supply device shown in FIG. 2 is an example of a configuration in which a predetermined power supply voltage (helix voltage Ehel) is supplied to electrode cathode 11 of traveling wave tube 1 shown in FIG. 1.
As shown in FIG. 2, the conventional power supply device comprises PWM (Pulse Width Modulation) control circuit 401 that outputs a pulse string matching a predetermined power supply voltage, switching element 402 which is turned on and off according to the pulse string outputted from PWM control circuit 401, and which generates a pulse string composed of a higher voltage, HV (High Voltage) transformer 403 boosting the pulse string outputted from switching element 402, rectifying circuit 404 rectifying the ac output of HV transformer 403, voltage detecting circuit 405 detecting an output voltage of rectifying circuit 404 and feeding a detected voltage Vfb, a result of the detection, back to PWM control circuit 401.
PWM control circuit 401 shown in FIG. 2 uses as a feedback voltage the detected voltage Vfb outputted from voltage detecting circuit 405 to adjust an output pulse width so that the feedback voltage remains constant, whereby the direct current voltage outputted from rectifying circuit 404 is kept constant.
In the above traveling wave tube 1, the order of application of voltages to electrodes should be controlled for preventing a situation in which an excessive current passes through helix 20 and damages helix 20 at the time that the power is turned on. The rise time of a helix voltage Ehel (voltage applied to electrode cathode 11 on the basis of grounded helix 20) should be reduced.
This is a measure required for preventing helix 20 from being damaged by the generation of heat associated with power consumption because electrons emitted from electrode cathode 11 are fed back to power supply device 60 through helix 20 without being captured by electrode collector 30 in a transient state in which the voltage applied to each electrode does not reach a predefined value.
In order to solve this problem, a power supply device comprising a sequence control circuit for controlling the order of application of voltages to electrodes is proposed, for example, in Japanese Patent Laid-Open No. 11-149880 (hereinafter referred to as Patent Document 1).
Patent Document 1 describes a method of controlling the introduction and blocking of anode voltages by using a small lead relay in a traveling wave tube in which connection between the electrode anode and the helix is cut.
In the traveling wave tube in which connection between the electrode anode and the helix is cut, as described above, if the electrode cathode and the electrode anode are set to have the same potential at the time of turning on the power, emission of electrons from the electrode cathode is prevented, thus making it possible to inhibit a helix current in a transient state when the power is turned on. Thus, in the high voltage power supply device described in the above Patent Document 1, the power is turned on while the electrode cathode and the electrode anode are set to have almost the same potential (negative potential), and then a voltage, applied to the electrode anode, is changed to a predefined voltage that uses a small lead relay having a low withstanding pressure.
However, in a configuration in which electrode anode 40 is connected to helix 20 and a common power supply voltage (e.g. ground potential) is supplied, such sequence control cannot be performed. Therefore, the instant when the power is turned on, a potential difference occurs between electrode cathode 11 and electrode anode 40, and electrons are emitted from electrode cathode 11 to cause passage of a considerable current through helix 20. Thus, when the power for the traveling wave tube shown in FIG. 1 is turned on, the helix voltage Ehel increase should be limited to a specific time period so that excess current does not damage helix 20. It is desirable that this should also be considered in the traveling wave tube in which connection between electrode anode 40 and helix 20 is cut.
The conventional power supply device described above has a configuration in which the output voltage of rectifying circuit 404 is detected by voltage detecting circuit 405, and the detection result is fed back to PWM control circuit 401 in order to keep the power supply voltage constant as shown in FIG. 2, and therefore the rise time of the power supply voltage at the time that the power is turned on depends on characteristics of the elements constituting the power supply device shown in FIG. 2.
Thus, control and reduction of rise time of the power supply voltage at the time of turning on the power is difficult, and helix 20 may be damaged when the power is turned on. There are cases where helix 20 has an excessive load and, even if the helix 20 is not damaged, the performance of traveling wave tube 1 will be degraded which will result in instability of operations.